Wireless communication systems keep evolving to meet the needs for providing continuous and faster access to a data network. In order to meet these needs, a multi-carrier system that is capable of using multiple carriers for the transmission of data has been proposed. The use of multiple carriers is expanding in both cellular and non-cellular wireless systems.
A multi-carrier system may increase the bandwidth available in a wireless communication system according to a multiple of how many carriers are made available. For instance, dual-cell high speed downlink packet access (DC-HSDPA) has been introduced in the Release 8 specifications of the third generation partnership project (3GPP) for universal mobile telecommunication systems (UMTS). With this feature, base stations (also referred to as a Node-B) communicate to WTRUs over two distinct carriers simultaneously.
For dual cell operation, each WTRU is assigned an anchor downlink carrier, which carries all dedicated and shared control channels used for high speed downlink shared channel (HS-DSCH), enhanced dedicated channel (E-DCH), and dedicated channel (DCH) operations (e.g., fractional dedicated physical channel (F-DPCH), E-DCH HARQ indicator channel (E-HICH), E-DCH relative grant channel (E-RGCH), E-DCH absolute grant channel (E-AGCH), common pilot channel (CPICH), high speed shared control channel (HS-SCCH), and high speed physical dedicated control channel (HS-PDCCH)). In addition, the WTRU may be assigned a supplementary downlink carrier, which carries CPICH, HS-SCCH and HS-PDSCH for the WTRU. The uplink (UL) transmission remains on a single carrier as in the current systems. The HS-DPCCH feedback information is provided on this UL carrier to the Node-B and contains information for each downlink carrier.
FIG. 1 shows the conventional medium access control (MAC) architecture for DC-HSDPA operations. The MAC layer architecture of DC-HSDPA includes one hybrid automatic repeat request (HARQ) entity per HS-DSCH transport channel.
As data rates continue to increase in the downlink (DL) via the introduction of multiple carriers, the capacity of the UL carrier would be consumed with control channels, (e.g., transmission control protocol (TCP) positive acknowledgement (ACK)/negative acknowledgement (NACK), radio link control (RLC) ACK/NACK and HARQ feedback). In order to increase data rates and capacity in the UL, it would be desirable to introduce dual cell or multi carrier uplink E-DCH transmissions. Since the achievable data rate grows linearly with the number of additional uplink carriers, E-DCH data rates of up to 23 Mbps could be achieved in dual-carrier high speed uplink packet access (HSUPA), for example.
The E-DCH, introduced in Release 6 of the specifications of the 3GPP, is based on a grant mechanism. At a high level, each Node-B has a means to control the level of interference caused by WTRUs by increasing or decreasing the serving grant. The serving grant represents the amount of power a WTRU is allowed to use for the E-DCH, which translates to a data rate. The amount of grant a WTRU gets is determined by the network based on the system load and the traffic offered. To help the network make suitable decisions, the WTRU provides buffer and power headroom information to the serving Node-B via scheduling information (SI) and happy bit.